Magnetoresistive head

ABSTRACT

A magnetic read head has a first embodiment comprising an elongated magnetoresistive element having a central region and distant ends. The central region has equipotential strips disposed intermediate to its ends, and detection circuitry is electrically connected to these intermediate equipotential strips to sense the changing resistance of the central region in the presence of data magnetically recorded on a medium. In a second embodiment, the magnetoresistive element is folded into a picture frame shape and has its ends joined. The element is vertically arranged so that one of the legs of the element is positioned in proximity to a selected track of a recording medium. A pair of equipotential strips are disposed at opposite ends of the leg to define a sensing region therebetween. Detection circuitry is connected to these equipotential strips to detect the changing resistance of the sensing region in the presence of the magnetic fields of the medium.

BACKGROUND

The present invention relates to magnetoresistive heads, examples ofwhich are shown in U.S. Pat. Nos. 4,040,113; 4,141,051; 4,052,748;3,848,217; 4,142,218; 3,979,775; 4,103,315; 4,315,291; 3,493,694;3,405,355; 4,321,640; and 3,860,965.

Magnetoresistive heads include a strip-shaped element of aferromagnetic, metallic, magnetically anisotropic material, for exampleNiFe, commercially known as Permalloy, which is deposited in a thin filmon a substrate and positioned either with one of its edges in theimmediate proximity of a magnetic recording medium, or alternatively,the element is positioned remotely from the medium with a flux guidearranged to bring the magnetic fields of the medium to the element. Thefields of the recording medium produce variations in the magnetizationof the element and thereby modulate the resistance of the element viathe magnetoresistive effect. In order to measure the changing resistanceof the magnetoresistive element, the element is electrically biased.This is typically done by directing an electric current through theelement. Detection circuitry is then connected to the element so thatthe changing resistance of the element can be monitored to produce anoutput which is representative of the information stored on the medium.

A problem associated with prior art magnetoresistive heads has been thepresence of Barkhausen noise in the output of the heads caused by theerratic movement of magnetic domain walls in the magnetoresistiveelement in response to the magnetic fields of the medium.

Another problem has been to ensure that the magnetic field generated bythe bias current in the magnetoresistive element does not become sogreat as to alter the magnetically recorded data on the medium.

SUMMARY

The magnetoresistive head of the present invention is, therefore,designed to eliminate Barkhausen noise by producing single domainmagnetization in the read portion, or active region, of themagnetoresistive element. In addition, the element is designed torequire only a minimal bias current to produce single domainmagnetization in the active region of the element.

The invention achieves this object by employing an elongatedmagnetoresistive element wherein only the central region of the elementis utilized for reading the data. By moving the ends of themagnetoresistive element far away from the central "active region" ofthe element, the demagnetizing effects of the magnetic fields at theends of the element on the central region of the element are minimized,and a minimal amount of current is, therefore, required to achieve asingle domain magnetic orientation in the central active region. Byproducing a single domain active region, Barkhausen noise caused by themovement of domain walls is eliminated since the domain walls arethemselves eliminated in the active region. In this "hammer head"embodiment, the changing resistance of the element is sensed only acrossthe active region rather than across the entire element. Equipotentialstrips are disposed on opposite ends of the active region and directlyconnected to the detection circuitry to facilitate this selectivereading of the element.

In a second embodiment, the ends of the elongated magnetoresistiveelement are, in effect, joined in that the magnetoresistive element isfolded into an endless vertically oriented frame. In this "pictureframe" embodiment, the magnetoresistive frame element has two horizontallegs and two vertical legs, with only one of the horizontal legs of theframe being used to read data. The demagnetizing forces produced by theends of the element are eliminated since the ends themselves have beeneliminated, and only a minimal amount of bias current is, therefore,required to achieve single domain magnetization in the fourmagnetoresistive elements of the frame.

It is, therefore, an object of the present invention to provide animproved magnetoresistive read head.

It is another object to provide a magnetoresistive read head wherein themagnetic flux path of the element is substantially longer than thecentral active region of the element employed for reading data from themedium.

Another object is to provide an elongated magnetoresistive read headwherein a minimal current is required to achieve a single domainorientation in the central active region of the element used for readingdata.

Still another object is to provide a magnetoresistive read elementwherein the changing resistance of the central active region of theelement is measured between equipotential strips disposed on oppositeends of the active region.

Yet another object is to effectively provide a very longmagnetoresistive read element in order to minimize the effect of thedemagnetization forces produced by the ends of the element.

Still another object is to effectively provide a very longmagnetoresistive read element by folding the element and joining itsends.

These and other objects, advantages and novel features of the inventionwill become apparent from the following detail description of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A shows the hammerhead embodiment of the present invention.

FIG. 1B shows a portion of the FIG. 1A embodiment.

FIG. 1C shows an unmagnetized magnetoresistive strip.

FIG. 1D shows a magnetoresistive strip under the influence of anexternal magnetic bias.

FIG. 2 shows the picture frame embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The basic principal of the present invention is most easily describedwith reference to FIGS. 1A through 1D.

FIG. 1A shows an elongated magnetoresistive element 1 such as would bedeposited on a substrate (not shown) and incorporated into a magnetichead (not shown) to read a selected track 5 of the magnetic recordingmedium 10. Element 1 has a distant first end 15, and a distant secondend 17, and a central region 19 intermediate the distant ends 15, 17.Equipotential strips 23, 25, 27, 29, 31, 33, 35, 37 and 39 are disposedat acute angles with respect to the lower edge 41 of the element 1. Forexample, strip 25 is disposed at acute angle 26 with respect to loweredge 41 of the element 1 as shown. Typically, the strips 23, 25, 27, 29,31, 33, 35, 37 and 39 would be disposed at 45 degree angles across theelement 1, but other acute angles could also be employed. A currentsource 45 drives current through the element 1 from its input 47 to itsoutput 49. With the current flowing from left to right in FIG. 1A, it iswell known that the current will leave and enter each equipotentialstrip 23, 25, 27, 29, 31, 33, 35, 37 and 39 in a direction normal to theedge of the strip. Thus, for example, the current will leave strip 29and enter strip 31 along vector 53 in FIG. 1A. Vector 53 has a vectorcomponent 55 which is parallel to the longitudinal axis of element 1 anda vector component 57 which is transverse to the longitudinal axis ofelement 1. These vectors are better shown in FIG. 1B. It is known thatthe easy direction of magnetism in a narrow elongated magnetoresistiveelement is along the longitudinal axis. It is also known that byapplying the right hand rule to transverse vector 57 we can determinethat the magnetic flux generated by the vector 53 will be in thedirection of vector 61. To understand the effect of this magnetic bias,reference is first made to FIGS. 1C and 1D. FIG. 1C shows anunmagnetized elongated strip of magnetoresistive material. In theunmagnetized state, the strp is multidomain, and in this example thestrip would be divided into four magnetic domains directed along thevectors 65, 67, 69, and 71, respectively. If we apply an externalmagnetic field along the direction of vector 75 in FIG. 1D, however, wesee that the magnetic domain parallel to the direction of the externalfield 75 grows, and that at a certain level of external magnetic fieldbias, the central region of the strip becomes single domain magnetizedin the direction of vector 65. The end regions remain multidomain. Theamount of magnetic bias necessary to produce single domain magnetizationin the central region is a function of the demagnetizing forces exertedon the central region by the ends of the element. Therefore, the longerwe make the element, the less bias current is required to produce singledomain magnetization in the central region. When the central regionbecomes single domain, domain walls in the region are eliminated, andtherefore, Barkhausen noise is eliminated.

Hence, returning to FIGS. 1A and 1B, if we consider all of thetransverse components of the current vectors for the element 1, we canappreciate that, at a certain level of external bias current, the cenralregion 19 of the element 1 will become single domain magnetized in thedirection of vector 61. Since the ends 15 and 17 are distant from region19, only a relatively small biasing current is required to overcome thedemagnetizing effect produced by the ends 15,17 on the central region 19in order to make region 19 single domain. Region 19 is bounded byequipotential strips 29 and 35. As shown in FIG. 1A, voltage dropsensing circuitry 79 is connected by electrical contact 81 toequipotential strip 29 and by electrical contact 85 to equipotentialstrip 35. The resistance of central region 19 fluctuates in response tothe magnetic fields recorded on track 5 of the medium 10. The responseof the single domain magnetoresistive region 19 to the fluctuatingfields of track 5 is superior to that of a multidomain region sinceBarkhausen noise is eliminated as noted. The changing resistance ofcentral region 19 in response to the recorded data is sensed by circuity79 to generate an output voltage representative of the data.

Having disclosed a first embodiment of the invention, the secondembodiment of the invention is shown in FIG. 2.

FIG. 2 shows a folded, or endless, magnetoresistive element 90. Element90 has a square, or rectangular, "picture frame" shape, and is comprisedof first vertical leg 92, first horizontal leg 94, second vertical leg96, and second horizontal leg 98. THe element 90 has corners 100, 102,104 and 106 as shown. A first equipotential strip 110 is disposed acrosscorner 100, having a first edge 112 angled across leg 92 and a secondedge 114 angled across leg 94. A second equipotential strip 116 isdisposed across corner 102 having a first edge 118 angled across leg 94and a second edge 120 angled across leg 96. A third equipotential strip122, having edges 124, 126 is angled across leg 96. A fourthequipotential strip 128 is disposed across corner 104 having first edge130 angled across leg 96 and second edge 132 angled across leg 98. Afifth equipotential strip 134 having edges 136, 138 is angled across leg98. A sixth equipotential strip 140 is disposed across corner 106,having a first edge 142 angled across leg 98 and a second edge 144angled across leg 92. The respective edges of the equipotential strips110, 116, 122, 128, 134 and 140 cross legs 92, 94, 96 and 98 at anglesof approximately 45 degrees in FIG. 2. The crossing angles of the stripscould, however, be varied depending on the results desired, andaccordingly, the invention is not intended to be limited to a 45 degreedisposition of the equipotential strips. A current source 150 has aninput 152 electrically connected to first equipotential strip 110 and anoutput 154 electrically connected to second equipotential strip 116.Voltage drop detection circuit 158 has an electrical connection 160 tofirst equipotential strip 110 and an electrical connection 162 to secondequipotential strip 116. There are two current paths through the element90 between current source input 152 and current source output 154. Thefirst current path goes from strip 110 through leg 94 to strip 116. Thesecond current path goes from strip 110 through leg 92 to strip 140,from strip 140 through leg 98 to strip 134, from strip 134 through leg98 to strip 128, from strip 128 through leg 96 to strip 122, and fromstrip 122 through leg 96 to strip 116.

The first current path described above passes through leg 94 from strip110 to strip 116 in the direction of vector 166 due to the fact thatcurrent enters and leaves equipotential surfaces in a direction normalto the equipotential surfaces as previously noted. Vector 166 haslongitudinal component 168 and transverse component 170. Applying theright hand rule to transverse component 166 we see that the direction ofthe magnetic field generated in the first horizontal leg 94 is in thedirection of vector 172.

Turning to the second current path described above, the current leavesstrip 110 in the direction of vector 174. Vector 174 has longitudinalcomponent 176 and transverse component 178. Applying the right hand ruleto transverse vector 178, the magnet flux in leg 92 will be in thedirection of vector 180. The current will leave equipotential strip 140in the direction of vector 182. Applying the right hand rule to thetransverse vector component 186, leg 98 will be magnetized in thedirection of vector 188. It is obvious that the same analysis applied tothe current leaving strip 134 along vector 190 would indicate adirection of magnetism along vector 192. Current would leaveequipotential element 128 in the direction of vector 194. Applying theright hand rule to the transverse component 196 indicates a direction ofmagnetism in leg 96 along vector 198. Applying the same analysis to thevector 200, which represents current leaving strip 122, the direction ofmagnetic flux is along vector 202.

It can, therefore, be appreciated that the legs 92, 94, 96 and 98 of thepicture frame element 90 are magnetized in a counterclockwise directionin FIG. 2 as indicated by the vectors 172, 202, 198, 192, 188 and 180.Hence, the magnetic fields of the legs 92, 94, 96 and 98 are designed toaid one another by virtue of the placement of the equipotential strips110, 116, 122, 128, 134 and 140 and the connections of the biasingcurrent source 150 to the elements 110 and 116.

The element 90 is, in effect, an elongated element having its endsjoined at the middle of the second horizontal leg 98. In that thepicture frame magnetoresistive element 90 is folded into an endlessframe with its ends being, in effect, joined, there are no ends of theelement to produce multidomain demagnetizing forces. Hence, the legs 92,96, and 98, and particularly 94, are converted to single domainmagnetization udner a relatively small bias current. Leg 94 is designedto have no more than one tenth of the resistance of the current paththrough legs 92, 96 and 98, and therefore, carries at least ten times asmuch current as passes through the legs 92, 96, 98. As shown in FIG. 2,the element 90 is vertically arranged with respect to the recordingmedium 10 and only the lower leg 94 of element 90 is disposed inproximity to the selected data track 5. The portion of leg 94 betweenthe second edge 114 of strip 110 and the first edge 118 of strip 116 isthe sensing region 210 of element 90 in that the voltage drop detectioncircuitry 158 sense the voltage drop across the region 210 fromequipotential element 110 to equipotential element 116. As data track 5moves past the element 90 (or out of the page normally in FIG. 2), theresistance of the sensing region 210 of the magnetoresistive element 90varies in response to the magnetic fields recorded on the medium 10.This variation in the resistance of region 210 is detected by thecircuitry 158 and converted into a suitable output which isrepresentative of the data on the medium.

As was the case with the elongated element 1, the relatively small biascurrent required to convert the element 90 into a single domainmagnetization, generates a relatively small magnetic field in the leg 94of element 90 so that there is no danger that the data recorded on themedium will be altered. Moreover, since sensing region 210 is singledomain, Barkhausen noise is eliminated in the output.

It was previously noted that there are two current paths through theelement 90. As noted above, the element 90 is designed so that thecurrent path directly through leg 94 has no more than one tenth of theresistance of the current path through legs 92, 96, 98. Consequently,the current through the "read leg" 94 is at least 10 times greater thanthe current through the legs 92, 96, 98, which increases the sensitivityand read capability of the element 90.

Having disclosed two embodiments of the invention, many variations andmodifications thereof would be obvious to those skilled in the art inview of the teachings herein, and the invention is therefore intended tobe limited only by the scope of the appended claims.

We claim:
 1. A magnetic head for detecting information representingmagnetic fields on a selected track of a magnetic recording medium,comprising:an elongated magnetoresistive element having a distant firstend and a distant second end opposite thereto, and a central regionintermediate to said distant first and second ends; a means forgenerating a magnetic field within said element; and a means fordetecting a change in resistance of said magnetoresistive element acrosssaid central region of said element as said magnetic fields of saidrecording medium are presented to said central region, said detectionmeans excluding any changes in resistance of the ends of said element.2. The magnetic head of claim 1, wherein a first equipotential strip isdisposed at an acute angle across said element, and a secondequipotential strip is disposed of an acute angle across said element,said first and second equipotential strips being disposed at oppositeends of said central region of said element whereby said central regionis defind by the region of said element between said first and secondequipotential strips, said detection means including circuitryoperatively associated with said first and second equipotential stripsto sense the change in resistance of said central region of saidmagnetoresistive element.
 3. The magnetic head of claim 2 wherein saidmagnetic field generating means comprises a means for forcing currentthrough said magnetoresistive element and said detection means compriseselectrical contacts connected from detection circuitry of said detectionmeans to said first and second equipotential strips, said detectioncircuitry measuring the voltage drop across said central region of saidelement between said first and second equipotential strips.
 4. Themagnetic head of claim 1 wherein said magnetic field generating meanscomprises a means for forcing current through said magnetoresistiveelement, and wherein said current flowing through said element issufficient to produce a single domain magnetic orientation in saidcentral region.